The present invention relates to a knee joint endoprosthesis, and particularly as a total replacement implant.
Knee joint endoprostheses of this type are known in many embodiments, and are already in clinical use. Fundamental efforts have been made to emulate the natural, anatomical sequence of motion in such an artificial joint, in order, for example, not to place an excessive strain on the system of ligaments, insofar as these remain intact.
Known anatomical knee joint endoprostheses usually include a femoral part connectable with a shank part that is to be anchored in the femur bone. This femoral part is provided with two runners (sliding parts), which are connected ventrally with one another via a bridge. There are also embodiments in which the runners are fashioned in one piece with the shank part. However, within the scope of the present invention it is not important whether the femoral part has a one-part or a multi-part construction.
In addition, an anatomical knee joint endoprosthesis usually includes a tibia plateau part which works together with a shank part that is to be anchored in the tibia bone. This tibia plateau part is preferably made of a plastic, such as high-density polyethylene, in which two slide guides are formed, on which the runners of the femoral part can execute a roll-off and sliding movement.
Corresponding to the pre-existing physiological conditions in such an anatomical knee jointxe2x80x94starting from the extended position of the jointxe2x80x94as the bending of the joint increases, the possibility becomes greater that the femoral part rotates around the tibia part. At the same time, the femoral part executes a drawer-like movement in the dorsal direction on the tibia plateau part.
Other designs do not take a purely anatomical approach, but instead attempt to meet the needs of the patient through a purely technological approach. Thus, for example in DE 100 60 850, which was not prior published, applicant has proposed a set for constructing a tibia part of an above-mentioned knee joint endoprosthesis, comprising a base (or pedestal) part as well as at least two plastic supports that can be seated loosely on the base part. The slide guides for the runners of a femoral part of the endoprosthesis are formed in these plastic supports. The slide guides of the second plastic support are displaced relative to those of the first plastic support from the ventral side toward the dorsal side by a distance that can be between 0 and 12 mm. With this set the condition of the patient""s system of ligaments is in particular taken into account, and the surgeon will assemble from the set an implant tailored to the individual patient. As stated, this approach is purely technological, and offers a plausible alternative to anatomical knee joint endoprostheses.
The scientific discussion concerning the advantages and disadvantages of one solution or the other has not reached a final conclusion. For the respective patient under treatment, each approach has advantages and disadvantages in accordance with the indications of the case.
Against this background it is now an object of the present invention to further develop a knee joint endoprosthesis of the type described at the outset, in such a manner that aspects of both the purely anatomical knee joint endoprosthesis and of the purely technological endoprosthesis are realized, in order to combine the respective advantages of both types of endoprosthesis in one.
This object is achieved by a knee joint endoprosthesis having:
a femoral part connectable with a femur shank part to be anchored in a femur bone, the femoral part being provided with two runners connected with one another ventrally via a bridge, and
a tibia plateau part working together with a tibia shank part to be anchored in a tibia bone, in which tibia plateau part there are formed two slide guides on which the runners of the femoral part can execute a rolling-off and sliding movement, wherein the tibia plateau part is pivotably mounted on the tibia shank part by a rotation mechanism, a negative and positive fit between the femoral part and the tibia plateau part being produced up to a predetermined bending angle of the knee joint, such that a rotational movement of the femoral part on the tibia shank part can be executed due to the pivotable mounting of the tibia plateau part on the tibia shank part, and wherein the femoral part is enabled to execute an increasing rotation of the femoral part relative to the tibia plateau part as the bending of the knee joint increases starting from the predetermined bending angle, without activating the rotation mechanism between the tibia plateau part and the tibia shank part.
Advantageous further developments are set forth below and in the dependent claims.
It is fundamentally proposed that the tibia plateau part be pivotably mounted on the tibia shank part so as to be able to be pivoted by a rotation mechanism and, up to a predetermined bending angle of the knee joint, effect a force- and form-fit (negative and positive) connection between the femoral part and the tibia plateau part, such that the femoral part can execute a rotational movement on the tibia shank part in the tibia bone due to the pivotable mounting of the tibia plateau part on the tibia shank part. On the other hand, an increasing rotation of the femoral part relative to the tibia plateau part is enabled as the bending of the knee joint increases starting from the predetermined bending angle, without, however, activating the rotation mechanism between the tibia plateau part and the tibia shank part.
Starting from the extended position of the knee joint, as the knee first bends, the motion sequence of a technological knee joint is at first most evident. Here, before a particular bending angle has been exceeded, the negative and positive fit between the femoral part and the tibia plateau part catches hold, and the purely technologically conditioned possibility of rotational movement, due to the mounting of the tibia plateau part on the tibia shank part, permits the femoral part to execute a rotational movement relative to the tibia shank part by means of a rotation mechanism. Thus, until a certain bending angle has been exceeded, the femoral part carries the tibia plateau part along in its movements and executes, as a unit therewith, a rotational movement on the tibia shank part anchored to the tibia bone. Subsequently, upon exceeding the predetermined bending angle, a regular switching takes place inside the endoprosthesis, between the previous technologically conditioned sequence of motion and the purely anatomical sequence of motion that then comes to bear. For this reason, this joint is also designated a hybrid joint, because it unites the anatomical and the technological aspects of known endoprostheses.
In a concrete preferred embodiment, it is provided that the runners of the femoral part delimit between them an open space that runs from the ventral side to the dorsal side, and that the two slide guides of the tibia plateau part are separated by two webs running aligned from dorsal to ventral. Here, the dorsally situated web has a height such that, up to the predetermined bending angle of the knee joint, it engages in the open space between the runners of the femoral part, thus creating the negative and positive fit. The ventrally situated web, in contrast, has only such a height that, as the bending of the knee joint increases, beginning with the exiting of the dorsal web from the open space between the slide guides of the femoral part, i.e., after the switch between technologically conditioned movement and anatomical movement, an increasing rotation of the femoral part is enabled relative to the tibia plateau part, but without activation of the rotation mechanism. This means that the ventrally situated web has a very low height and primarily exercises a guide function.
In this specific embodiment, the xe2x80x9cswitchingxe2x80x9d is realized by the femoral part and by the tibia plateau part, in the one case by the open space between the runners having corresponding support surfaces, and in the other case by the formation of the dorsally situated web, whose side walls come into contact with the side edges of the condyles of the femoral part and transmit the flow of force.
In a further preferred development the predetermined bending angle for a tibia plateau part lies in a range of about 20xc2x0 to about 40xc2x0, the negative and positive fit between the femoral part and the tibia plateau part being effective up to the point at which this angle is exceeded. The xe2x80x9cswitchingxe2x80x9d of the knee joint thus takes place in the indicated range of about 20xc2x0 to about 40xc2x0. The actual termination of the negative and positive fit as the bending of the joint increases depends decisively on the height of the dorsally situated web of the tibia plateau part, and can be determined individually for each patient.
According to an advantageous further development, it is also possible for the ventrally situated web to transition into the dorsally situated web. From the point of view of manufacturing technology, this is a simplification.
According to an advantageous embodiment, the rotation mechanism between the tibia plateau part and the tibia shank part comprises a pin that is integrally formed on the tibia shank part and that points towards the femur, and a correspondingly dimensioned bore in the underside of the tibia plateau part, in which the pin engages. This type of pivotability of a tibia plateau part on the tibia shank part is known per se.